Kaposi's sarcoma herpesvirus (KSHV) causes Kaposi's sarcoma (KS) and two lymphoproliferative disorders: primary effusion lymphoma (PEL) and multicentric Castleman's disease (MCD). The risk for KS is greatly increased in HIV-infected individuals and this is the most common malignancy in several countries in subequatorial Africa. KS is largely incurable with current therapeutic options. While KS is caused by KSHV, no effective virus-specific therapies exist. A better understanding of the pathobiology of KSHV infection, and robust animal models are essential for developing preventive and therapeutic strategies. Current animal models rely on expression of single viral genes in broad cellular populations, use of related animal viruses (none of which lead to KS or PEL-like disease), xenografts consisting of human tumor cells injected into immunodeficient mice, infection of humanized mice (with no resulting pathology), or implantation of cells previously infected or transfected in vitro. All of these animal models have significant limitation. Therefore, the goal of this project is to generate better mouse models to evaluate the effect of KSHV latent viral genes in vivo when selectively expressed in the specific cellular compartments that comprise the tumor cells in PEL and KS, namely B cells and endothelial cells. The majority of the cells in KSHV-associated tumors express only a few latent viral genes, although a very small and variable number of cells may also express some lytic genes. Conditional knock-in mice expressing the latent viral gene vFLIP in two different B cell subsets, all CD19+ cells or germinal center B cells, develop lymphadenopathies with features of MCD and tumors of B cell origin with long latency. We propose to extend these studies through the following specific aims: 1) discover complementing cellular and viral genetic events in vFLIP-mediated lymphomagenesis; 2) develop and characterize mice expressing inducible vFLIP in endothelial cells; and 3) determine the effect of expression of the entire KSHV latency locus in endothelial cells, and assess the contribution of vGPCR expression in rare scattered cells. Through these aims, we anticipate developing mouse models that resemble human disease, which will be useful for a deeper understanding of the mechanisms of KSHV pathogenesis, and testing novel therapeutic approaches.